1. Field of the Invention
The present invention relates to a semiconductor laser device having an end-facet window structure, i.e., a structure which makes an end facet nonabsorbent of oscillation light.
2. Description of the Related Art
J. K. Wade et al. (xe2x80x9c6.1 W continuous wave front-facet power from Al-free active-region (xcex=805 nm) diode lasers,xe2x80x9d Applied Physics Letters, vol. 72, No. 1 (1998) pp.4-6) disclose a semiconductor laser device which emits light in the 805 nm band. The semiconductor laser device comprises an Al-free InGaAsP active layer, an InGaP optical waveguide layer, and InAlGaP cladding layers. In addition, in order to improve the characteristics in the high output power range, the semiconductor laser device includes a so-called large optical cavity (LOC) structure in which the thickness of the optical waveguide layer is increased so as to reduce the light density, and increase the maximum light output power. However, when the optical power is maximized, currents generated by optical absorption in the vicinity of end facets generate heat, i.e., raise the temperature at the end facets. In addition, the raised temperature reduces the bandgap at the end facets, and therefore the optical absorption is further enhanced to damage the end facet. That is, a vicious cycle is formed. This damage is the so-called catastrophic optical mirror damage (COMD). When the optical power reaches the COMD level, the optical output deteriorates with time. Further, the semiconductor laser device is likely to suddenly break down due to the COMD. Therefore, the above semiconductor laser device is not reliable when the semiconductor laser device operates with high output power.
In addition, T. Fukunaga et al. (xe2x80x9cHighly Reliable Operation of High-Power InGaAsP/In0.48Ga0.52P/AlGaAs 0.8 xcexcm Separate Confinement Heterostructure Lasers,xe2x80x9d Japanese Journal of Applied Physics, vol. 34 (1995) L1175-L1177) disclose a semiconductor laser device which comprises an Al-free active layer, and emits light in the 0.8 xcexcm band. In the semiconductor laser device, an n-type AlGaAs cladding layer, an intrinsic (i-type) InGaP optical waveguide layer, an InGaAsP quantum well active layer, an i-type InGaP optical waveguide layer, a p-type AlGaAs cladding layer, and a p-type GaAs cap layer are formed on an n-type GaAs substrate. S. O""Brien, H. Zhao, and R. J. Lang report, in Electronics Letters, vol. 34, No. 2 (1998) p.184, that the maximum output power of the above semiconductor laser device disclosed by Fukunaga et al, is 1.8 W. They also report the maximum breakdown light output power of multiple-transverse-mode semiconductor laser devices having a stripe width of 50 micrometers or greater. For example, at the wavelength of 0.87 micrometers, the maximum breakdown light output power of a multiple-transverse-mode semiconductor laser device having a stripe width of 100 micrometers is reported to be 1.3 W, and the maximum breakdown light output power of a multiple-transverse-mode semiconductor laser device having a stripe width of 200 micrometers is reported to be 16.5 W.
Further, the present inventor, T. Hayakawa, and others report, in Applied Physics Letters, Vol. 75, No. 13 (1999) p. 1839, that the practical light output power of 1.5 W is achieved in continuous oscillation of a semiconductor laser device having a stripe width of 50 micrometers when the semiconductor laser device is designed to increase the beam width in the direction perpendicular to the active layer, lower the peak optical strength, and minimize the temperature raise at a light-exit end facet. However, it is difficult to increase the reliability and the practical light output power of the semiconductor laser device by a large amount.
In order to solve the above problems, the Japanese Patent Application No. 11(1999)-348527 and the copending U.S. patent application Ser. No. 09/731,702, xe2x80x9cHIGH-POWER SEMICONDUCTOR LASER DEVICE IN WHICH NEAR-EDGE PORTIONS OF ACTIVE LAYER ARE REMOVEDxe2x80x9d, corresponding to the Japanese patent application and being filed on Dec. 8, 2000 by Toshiaki Fukunaga and assigned to the same assignee as the present patent application, disclose a semiconductor laser device in which transparent regions are formed in vicinities of end facets with Al-free material. However, the transparent regions are required to be formed by crystal regrowth, and the crystal regrowth is initiated from a surface of an optical waveguide layer located near to the quantum well active layer. That is, portions of the regrowth boundary are near the quantum well. In addition, there is no energy barrier between the quantum well active layer and the other portions of the regrowth boundary. In the semiconductor laser device formed as above, carriers leaked from the active layer and diffused to the regrowth boundary cause non-radiative recombination at the regrowth boundary. Therefore, the efficiency of the semiconductor laser device decreases, and degradation is promoted. Further, before the regrowth, the regions in the vicinities of the end facets must be etched to the depth of a crystal layer which is located immediately below the active layer. Therefore, it is not easy to control the depth of the etching.
The object of the present invention is to provide a semiconductor laser device in which regions in vicinities of end facets are made of a material being non-absorbent of oscillation light so that non-radiative recombination at a regrowth boundary is prevented, and the reliability and performance of the semiconductor laser device are improved.
According to the present invention, there is provided a semiconductor laser device comprising a substrate, an active region formed above the substrate, and a non-absorbing layer formed over the active region, and made of a semiconductor material having a bandgap greater than the photon energy of laser light which oscillates in the semiconductor laser device. The active region includes a first lower optical waveguide layer formed above the substrate, an etching stop layer formed on the first lower optical waveguide layer except for near-edge areas of the first lower optical waveguide layer, a second lower optical waveguide layer formed on the etching stop layer, a quantum well active layer formed on the second lower optical waveguide layer, a first upper optical waveguide layer formed on the quantum well active layer, an electron barrier layer formed on the first upper optical waveguide layer and made of a semiconductor material having a bandgap greater than a bandgap of the first upper optical waveguide layer, and a second upper optical waveguide layer formed on the electron barrier layer, where the near-edge areas are located adjacent to opposite end facets which are perpendicular to the direction of the laser light. The etching stop layer has such a chemical property that the etching stop layer can be maintained when the second lower optical waveguide layer, the quantum well active layer, the first upper optical waveguide layer, and the electron barrier layer are etched, and the first lower optical waveguide layer can be maintained when the etching stop layer is etched.
When the semiconductor laser device according to the present invention is produced, first, a first lower optical waveguide layer, an etching stop layer, a second lower optical waveguide layer, a quantum well active layer, a first upper optical waveguide layer, an electron barrier layer, and a second upper optical waveguide layer are formed above the substrate in this order. Next, near-edge portions (i.e., portions in vicinities of end facets which are perpendicular to the light axis) of the second lower optical waveguide layer, the quantum well active layer, the first upper optical waveguide layer, the electron barrier layer, and the second upper optical waveguide layer are removed by selective etching. Then, near-edge portions of the etching stop layer are also removed by selective etching. The etching stop layer has such a chemical property that the etching stop layer can be maintained when the second lower optical waveguide layer, the quantum well active layer, the first upper optical waveguide layer, the electron barrier layer, and the second upper optical waveguide layer are etched, and the first lower optical waveguide layer can be maintained when the etching stop layer is etched. Therefore, the etching of the above near-edge portions can be stopped at the upper surface of the first lower optical waveguide layer with high accuracy. Thereafter, a non-absorbing layer, which is made of a semiconductor material having a bandgap greater than the photon energy of laser light which oscillates in the semiconductor laser device, is formed over the active region. Since the upper surface of the first lower optical waveguide layer which is apart from the quantum well active layer is a regrowth boundary, carriers leaked and diffused from the quantum well active layer do not cause non-radiative recombination at the regrowth boundary. Therefore, it is possible to prevent the decrease in the efficiency due to the non-radiative recombination and the degradation of the end facet due to heat generation. Thus, the performance and reliability of the semiconductor laser device can be improved.
In addition, since the electron barrier layer is formed between the first and second upper optical waveguide layers, and made of a semiconductor material having a bandgap greater than the bandgap of the first upper optical waveguide layer, it is possible to prevent leakage of carriers from the active layer to a regrowth boundary located above the second upper optical waveguide layer. Therefore, at the regrowth boundary, no non-radiative recombination is caused by the carriers leaked from the active layer. Thus, the decrease in the efficiency due to the non-radiative recombination and the degradation of the end facet due to heat generation can be prevented.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) and (ii).
(i) The quantum well active layer may be made of an aluminum-free semiconductor material. It is well known that when an active layer does not contain aluminum, composition change due to oxidation of aluminum can be prevented, and the reliability of the semiconductor laser device can be increased. However, in particular, when the quantum well active layer in the semiconductor laser device according to the present invention is made of an aluminum-free semiconductor material, the reliability of the semiconductor laser device can be remarkably increased.
(ii) The non-absorbing layer and a semiconductor layer immediately under the non-absorbing layer may be made of an aluminum-free semiconductor material. In this case, both of the regrown layer and the base layer of the regrowth are aluminum-free. Therefore, the reliability of the semiconductor laser device can be remarkably increased.
FIGS. 1A, 1B and 1C are cross-sectional views of a semiconductor laser device as the first embodiment of the present invention.
FIG. 2 is a cross-sectional view of a semiconductor laser device as the second embodiment of the present invention.
FIG. 3 is a cross-sectional view of a semiconductor laser device as the third embodiment of the present invention.
FIG. 4 is a cross-sectional view of a semiconductor laser device as the fourth embodiment of the present invention.
FIGS. 5A, 5B and 5C are cross-sectional views of a semiconductor laser device as the fifth embodiment of the present invention.
FIGS. 6A, 6B and 6C are cross-sectional views of a semiconductor laser device as the sixth embodiment of the present invention.